1. Field of the Invention
The present invention relates to apparatus for neutralizing electrostatic charges, on charge-retaining materials, in general, and to such apparatus for neutralizing electrostatic charges on a moving web of such material, in particular.
2. Description of the Prior Art
The presence of electrostatic charges on charge-retaining materials causes problems in many industries. In the photographic industry, for example, electrostatic charges on potential photographs or film units within a light-tight film cassette containing a plurality of film units for use in an "instant" type photographic camera, such as that sold by Polaroid Corporation, Cambridge, Massachusetts, under its registered trademark "SX-70", will often cling to one another with such intensity as a result of the force of attraction developed by such electrostatic charges, that proper ejection of an exposed film unit from said film cassette can be prevented if the effects of such charges are not controlled. In the SX70 photographic film mentioned above, for example, electrostatic charges are controlled by controlling the charge levels on components of said film prior to final film assembly.
Troublesome electrostatic charges on charge-retaining materials can be conveniently grouped into either of two catagories. One catagory is that of polarization charges, sometimes referred to a dipoles, and the other is that of free surface charges. Polarization charges are bound to a definite site in a solid, whereas free surface charges are not. Free surface charges on a moving web of certain materials, for example, are frequently neutralized by a grounded brushlike device such as that described in U.S. Pat. No. 3,575,164 to BINKOWSKI. Polarization charges in such a web are commonly controlled by subjecting the web to a corona-generated electrostatic field having a particular magnitude and polarity. It is often necessary to deal with both categories of charges.
The polarity of polarization charges or dipoles on charge-retaining materials may be positive, negative, or a combination of both. If both positive and negative charges should be present in the same charge-retaining material and if said positive and negative charges are to be neutralized by having their charge levels reduced to zero, the application of a DC-type electrostatic field having either a positive or a negative polarity will not reduce the charge level of both positive and negative charges to zero. If, for example, a DC-type electrostatic field having a particular polarity is successfully employed to neutralize one kind of polarity charge (positive or negative), the opposite polarity charge from that which is neutralized will have its charge level undesirably increased and not neutralized or reduced to the zero level that is preferred.
Apparatus that includes a corona-generated electric field can be effectively employed to neutralize polarization charges. However, corona has several undesirable properties. A corona generates ozone gas and ozone can cause some individuals to become ill. Even if illness does not result from the ozone, it has a pungent odor that is objectionable to most people, especially in an environment where ventilation is relatively poor or nonexistent. When a corona is employed to control polarization charges on a web of light-sensitive material, means must be provided to prevent light produced by the corona from reaching and thereby damaging said material. In addition, it is often necessary to expend significant quantities of relatively costly electric power in order to generate and maintain a corona-type electrostatic field for charge neutralizing purposes.